The following projects relative to the hormonal regulation of luteal and follicular growth and regression will be undertaken: 1. Measurement of peripheral levels of FSH, LH, prolactin, estrogens and progesterone in pregnant rats, hamsters, and mice. 2. Restoration of follicular development by gonadotropins in the hypophysectomized hamster. 3. Comparison of divergent patterns of steroid and gonadotropins in pregnant, hysterectomized, pseudopregnant, and unilaterally ovariectomized hamsters. 4. Estrogen as a direct luteotropic hormone in pregnant mammals other than the rabbit. 5. Follicular kinetics in cyclic and pregnant animals evaluated by autoradiography (H3 thymidine incorporation).